1. Field of the Invention
This invention relates to the field of projection television receivers and displays in general, and in particular, to projection television receivers and displays utilizing imagers having random row access, for example liquid crystal on silicon (LCOS) imagers.
2. Description of Related Art
A liquid crystal imager, for example an LCOS imager, can have an aspect ratio of 4:3, in accordance with standard television receivers. However, such a receiver must also be capable of displaying a video program having a wide screen aspect ratio of 16:9. A 16×9 video program can be displayed on a 4:3 display, using a 4:3 LCOS imager, by performing a vertical format conversion which converts the 16:9 image to fit into three fourths (¾) the height of the 4:3 display.
An aspect ratio conversion to show a 16:9 program with a 4:3 imager might require, for example, converting a 1080 line image into an 810 line image. In order to do this, the typical vertical format converter will wait two line periods until two lines are present and will then use the two lines to generate one output line. This output will occur during one line period of the input signal. Next, the converter will use a third input line, along with the input lines already received, to generate a second output line during the next input line period. After that, the converter will use a fourth input line to generate a third output line. At this point, the converter must wait for another input line to be entirely read in, in order to continue. This wait or delay is a whole input line period caused by the vertical format converter performing a recalculation of the pixels for each line. Thus the converter, in reducing the number of lines, has naturally occurring dead line periods in its output during which no data is sent out to the imager.
Normally, this function requires a substantial memory capacity, for example one fourth (¼) of a frame, as well as sufficient memory bandwidth to write the frames in a normal frame period and read the output frames in three fourths (¾) of the normal frame period. There is always a need in developing video display systems to reduce memory requirements and bandwidth, so that circuits can be simplified and costs can be reduced.